Spinal cord injury (SCI)causes neuronal cell death combined with astroglial proliferation and inflammation associated with activation of microglia. Upregulation of cell cycle proteins occurs after central nervous system (CMS) trauma, and appears to contribute to apoptotic cell death of post-mitotic cells such as neurons and oligodendroglia. It also likely contributes to post-traumatic gliosis and microglial activation. Recent studies in our laboratory have shown significantly increased expression of many cell cycle proteins after SCI in rodents, with the proteins co-expressed in neurons showing caspase-3 activation and morphological features of apoptosis. Moreover, in several classical models of caspase-3 dependent apoptosis in primary neuronal cell cultures, injury is associated with upregulation of many of these same cell cycle proteins. In addition, studies by us and others indicate that inhibition of key cell cycle regulatory pathways reduces injury-induced cell death both in vitro and after acute brain injury. We recently found that treatment with a cell cycle inhibitor after SCI in rats markedly reduces lesion volumes and the surrounding glial scar; it also significantly improves motor functions following injury. The proposed studies are intended to address the following hypotheses: (1) SCI up-regulates key cell cycle constituents at both the mRNA and protein levels in neurons, oligodendroglia, astrocytes, and microglia and induces activation of the cell cycle pathways; (2) such upregulation and activation promotes apoptosis in neurons and oligodendroglia; (3) upregulation and activation of cell cycle proteins also contribute to proliferation of astrocytes as well as microglial activation and subsequent release of associated inflammatory factors; and (4) treatment with cell cycle inhibitors is neuroprotective, through mechanisms that include inhibition of caspase-dependent apoptosis in neurons and oligodendroglia, along with reduced glial activation, diminished release of microglial mediated inflammatory factors and attenuation of the SCI- induced immuneresponse. The specific aims are to demonstrate that: (1) a. SCI causes increased expression of critical cell cycle related genes/proteins (including c-myc, cyclin D1, CDK4, Rb and E2F5) and activation of cell cycle pathways (activation of CDKs and phosphorylation of Rb) in neurons and glia in a rat spinal cord contusion model; b. increased cell cycle protein expression/activation is associated with caspase- dependent apoptosis in neurons and oligodendroglia; c. upregulation of cell cycle pathways is also associated with proliferation of astroglia; d. induction of cell cycle pathways after SCI activates microglia and induces the release of related inflammatory factors; (2) a. central (intrathecal) administration of two structurally different cell cycle inhibitors (Flavopiridol or roscovitine) decreases cell cycle activation after SCI, thereby reducing subsequent neuronal and oligodendroglial cell death, reactive gliosis and microglial activation; b. central administration of Flavopiridol or roscovitine reduces lesion size and improves motor recovery following SCI;(3) a. systemic administration of Flavopiridol has dose-dependent effects on functional outcome and the systemic immune response; b. systemic administration of Flavopiridol decreases cell cycle activation after SCI, thereby reducing subsequent neuronal and oligodendroglial cell death, reactive gliosis and immune activation; c. therapeutically relevant, delayed systemic administration of Flavopiridol improves post-traumatic function and histological outcome.